jonnybischof wrote:
Anyways. Summing it up:
From what you've told us so far, you're not doing anything wrong, and have tested the whole setup with multiple parts each. Still nothing works. That is... highly unlikely...

Have you tested everything using one and the same 3D printer controller?

I only have one
The controller is the same, the relais is the same, the MosFET setup is YOURS and the same...

jonnybischof wrote:
Double and triple check the wiring. Make sure you have adequate wiring, make sure you have no GND loops. Maybe google up some drawings from other people and compare... You wouldn't believe it, but no electric circuit is too simple to make a mistake with it (my personal experience...)

True, happened also to me...
I doublechecked everything - also the solder joints are ok, the wiring is thick enough and the drawings already checked. I just changed one component at a time in a working system, so... if everything working with my 12V/10A PSU, why the hell not with the 12V/18A PSU or the 19.5V/16,6A PSU - it's just changing the supply, no other part of the system...

Anyway - as we say in Austria "Auf den Bush klopfen und schauen was drunter rauskommt" - wack a bush and look what´s comming out - I will check everything again this weekend...

These heatbeds can deviate from the nominal resistance quite a bit, which is why the "always add 30% reserve on top of your expected maximum load" rule is so important. I usually add 50% or more, just because a beefier PSU will have a longer lifespan and run cooler compared to a "just right" PSU running near its limits.

Sooo..... yesterday started to rebuild my heated bed.
New small 12V/18A Meanwell PSU, new silicone heater. First dry test with direct wiring approveed that it is working. Heating of the mat is fast and hot
Think this will work... first assumption: ah, 30min, than it´s up and running... what shall I say: failed assumption

Old silicon heater removed, new silicon heater mounted, cables in place...
Then I noticed that 0,75mm² cable could be too small for 18A... so today I will change it - also the connectors need to be changed - and there must be some covers for the PSU clamps.
Next, the built in sensor is a 3950 thermistor, so FW change need (table 1 to 11) or just exchange the sensor by my reliable EPCOS 100K.

So finally it will take 30 min - per day - over one week...

@jonny, thank you for your assistance so far.
Just a small additional question regarding your MosFET relais - I know the MosFET can handle lot more than 18/20A, but is it ok/safe to solder 2,5² cables to those small MosFET feet?

The bed takes 30 minutes to heat up? o.O
That would be awful because you have to cycle temperatures to remove prints, right?

No problem soldering these wires directly to the MosFETs pins. That is the best way you can do it.
0.75 mm2 wiring is too weak for 18 A. As a result you should lose a noticeable amount of power. With 2.5 mm2 wiring you should get faster heating, and cool cables (they might heat up by a few degrees, but shouldn not get hot to something like 50°C).

The bed takes 30 minutes to heat up? o.O
That would be awful because you have to cycle temperatures to remove prints, right?

No problem soldering these wires directly to the MosFETs pins. That is the best way you can do it.
0.75 mm2 wiring is too weak for 18 A. As a result you should lose a noticeable amount of power. With 2.5 mm2 wiring you should get faster heating, and cool cables (they might heat up by a few degrees, but shouldn not get hot to something like 50°C).

Sorry, no, I thought that witin 30 min the whole exchange is done and the printer is back on duty...
"Organized" the calbes right now in our workshop so today evening it will be soldering time again.

Soldered everything together, tested with updated Firmware becasue of the new temp sensor - got errors...
Why has this cheap stuff be so ... shitty...
Taking out the 3950 thermistor took again a forever ss this thing was damn good glued. Replaced it finally with the former EPCOS.

Currently testing - 70°C without issues, 90°C takes a few minutes, but works too.
The MosFET brick works also flawless.

Long term tests will prove the setup and my soldering skills...

Next: printing some covers for the PSU clamps - maybe with ABS??
Have to do a bit of cleaning and make everything nice, but finally mission accomplished...

If you have Kaptop tape laying around - that's the easiest way to get the PSU terminals isolated. Use 3-4 layers because the tape is very thin. As long as your PSU doesn't get warm when operating you can take PLA. It's a good isolator as long as it doesn't melt.

Oh, the PSU gets warm - but the bed too... tested today up to 100°C (acc. to the thermistor reading) - took some time, but works...
I placed some water at the bed surface but it does not start boiling, just evaporated slowly.
Seems that there are a few degrees missmatch, but I think that´s ok... maybe I can borrow a laser thermometer at work to check this.
Hope that there is a way to fix it via FW...

The temperature sensor is probably not at fault. It's normal at high temperatures to have a high drop between the sensor (probably placed underneath the heater, very close to the heating coils?) and the final surface...
At 100°C, this could easily be a drop of 10 K unless you have a metal build plate.

Note that at 100°C bed temperature you should absolutely make sure to have the build chamber well enclosed and insulated. You are wasting a lot of energy if that's not the case. And at this point I would also move any motors from the inside of the machine to the outside - they are not built for high ambient temperatures.

I just tested it yesterday to check if it can reach 100°C in reasonable time. But unerstood - there will be some drop anyway, so I´ll probably modify just the temp in the material settings by 10K and leave it as it is.
Side closure is already installed, top accordeon covers that follow the moves too, just the front door is missing.

We will see how long/reliable this new setup is...
Anyway, thanks again for your support